Recently, not only the driving condition of a vehicle can be detected, but also the driving environment surrounding the vehicle can be detected, such as the road on which the vehicle is driving and is scheduled to drive, the road condition, etc. Furthermore, an engine and/or an transmission and/or other equipment installed on the vehicle can be controlled with electric devices. Not only can a driver's operation be electrically controlled, but the control based on the driving environment can also be realized these days.
One example of a device used to detect the driving environment is a navigation device. This device detects a position of the vehicle by a GPS (Global Positioning System), which uses an artificial satellite or self navigation, and shows the vehicle's position and the scheduled course to the destination on an electronically displayed map. Adding to this, it memorizes information relating to each road and outputs information of the road according to the detected vehicle position or the scheduled course. By using the aforementioned information, the automatic transmission can be controlled to shift appropriately according to the driving environment.
One example of a control device for an automatic transmission controlled on the basis of information acquired by such a navigation device as mentioned above, is disclosed in Japanese Laid-Open Patent Application No. 10-78119. In this control device, a control area which contains an intersection is detected and set by a navigation device or the like, and a higher gear ratio is prohibited in the control device. For example, when a vehicle drives in an intersection or near an intersection ahead, a predetermined high gear is prohibited. That is, the automatic transmission is restrained to be in a lower gear. Consequently, good acceleration performance can be achieved.
According to the Japanese Laid-Open Patent Application No. 10-78119, the automatic transmission is shifted in turn to a lower gear following the vehicle's speed-down, since the control device prohibits a higher gear of the automatic transmission in a road condition, for example in the intersection, where the vehicle would accelerate after decelerating. When the vehicle is required to accelerate during or after passing through the control area such as the intersection, the vehicle is in a power-on condition, and after that the vehicle is in turn up-shifted to a higher gear in accordance with the vehicle's speed-up. In these deceleration-acceleration processes of the vehicle, the automatic transmission is under a coast condition where the automatic transmission is driven by an output shaft. Consequently, if the automatic transmission has a one-way clutch, for example for setting the 1.sup.st gear, the one-way clutch does not work, that is, it is free during the deceleration of the vehicle.
The one-way clutch has a structure in that, for instance, a plurality of sprags are set between an outer race and an inner race. There is also a roller type of one-way clutch which has a plurality of rollers instead of sprags. When a torque is applied in a predetermined direction from the outer race or the inner race of the one-way clutch, the sprags are locked up between the outer race and the inner race. The outer race and inner race then rotate together as one piece. On the contrary, when the torque is applied in the other direction, the torque can not be transmitted between the outer race and the inner race since the sprags are free between the outer race and the inner race. Consequently, since the one-way clutch works or is free in accordance with the direction of the torque, a gear shift such as to the 1.sup.st gear is achieved by using the one-way clutch.
If an automatic transmission having a one-way clutch in low gear, for example 1.sup.st gear, is controlled by the above-mentioned control device, the automatic transmission is under the coast condition by disengaging an acceleration pedal causing a decelerating operation in a predetermined controlled area such as an intersection. The one-way clutch is not only free in the 2.sup.nd gear, but also is free even if the automatic transmission is down-shifted to the 1.sup.st gear. The one-way clutch works when the vehicle's speed is nearly zero or creeping with the engine idling. However it does not work when the speed is higher than the aforementioned condition. Consequently, when the acceleration pedal is pushed under a condition where the speed is higher than the low speed mentioned above, the one-way clutch suddenly begins to work following the 1.sup.st down-shift and an increasing of engine revolutions.
That is, first, the outer race or the inner race relatively rotates against another counterpart without transmitting torque. Next, the torque transmitting direction is reversed and the outer race and the inner race lock up, then both races rotate together or stop as one piece. Then the torque can be transmitted between both races. This means that the inner race suddenly rotates in the reverse direction or stops against the outer race, and the rather high torque including the inertia of the inner race are applied to the outer race. Consequently, the applied torque to the outer race suddenly increases from zero. Then an abrupt shock occurs in the vehicle because of the sudden torque fluctuation to the output shaft of the automatic transmission.
The aforementioned control device has an advantage of improving the acceleration performance passing through an intersection. But on the contrary, in a predetermined area such as the intersection a driver could feel an inevitable shock by the sudden working of the one-way clutch. This could cause the ride quality of the vehicle to worsen.